1. Field of the Invention
The present invention relates to handover control in a multiple-hop mobile communication network, and more specifically to a method for selecting a base station to directly communicate with while a mobile terminal is moving in a multiple-hop wireless communication network such as a wireless sensor network. The present invention also relates to a mobile communication terminal and a base station for use in such a method.
2. Description of the Background Art
Conventionally, in a mobile communication system, so-called handover operation is done to change a base station to be connected when a mobile terminal moves while communicating. Generally, in handover operation, a mobile communication terminal selects and changes a base station to be connected on the basis of the strength and reception quality of electromagnetic waves received from plural neighboring base stations. As such handover technology, several methods are proposed, which will be described below.
In a handover method on the basis of the reception level disclosed in the Japanese patent laid-open publication No. 2000-102057, the idle handover control process classifies the reception qualities of a pilot channel for establishing synchronization into quality class 1 which is able to receive data without error, quality class 2 which is able to receive data with tolerable error and quality class 3 which is difficult to receive data with good quality, with decision values of 6 db, 4 db and 2 db allotted, respectively. Then, it is decided whether or not the reception quality of the pilot channel with which synchronization is presently established is lower than the reception quality of another pilot channel that is an alternative with which synchronization maybe established with a decision reference value set in a classified quality class of the pilot channel with which synchronization is present established. Only if the other pilot channel is higher in reception quality than the pilot channel with which synchronization is presently established, the handover operation is performed.
In the U.S. patent application publication No. 2002/008217 A1 to Hattori, a method is disclosed in which by obtaining the positional information of neighboring base stations by means of the Global Positioning System (GPS) and measuring the distances from a mobile terminal, the handover operation is performed to an appropriate base station. In a waiting status, on the basis of the GPS signal periodically received by a GPS receiver, the position information of the terminal is calculated by means of a position calculating function, and then on the basis of the position information of that terminal and the position information of the base station notified in the system information sent from the base station with which synchronization is presently established, the distance between the base station and the terminal is calculated by means of a distance calculating function. Further, the function of determining the necessity of handover compares the calculated distance with a threshold value notified from the base station. When the distance is larger than the threshold value, the handover from the current system to a different system is performed by a handover control function.
Another U.S. patent application publication No. 2005/0014515 A1 to Suzuki discloses a method into which the method taught by '057 Japanese patent publication with the method taught by Hattori. In Suzuki, a position information detector, a distance calculator and a moving direction and speed calculator obtain the distances between the mobile communication terminal and its plural neighboring base stations and the moving direction and speed of the mobile communication terminal, and further a transfer path quality detector obtains the degree of fading deterioration of respective neighboring base stations. Next, on the basis of the parameters thus obtained, an offset calculator calculates offset values to the received signal level of the respective neighboring base stations. Finally, on the basis of the calculated off set values and the received signal levels of plural neighboring base stations, a handover destination base station to which the communication is to be handed over is selected by means of a handover destination selector.
In these conventional handover control methods, the base station having the best communication condition is basically selected. In other words, the nearest base station is selected in order under a general circumstance. Because, these conventional handover control methods are principally applied to mobile telephone networks under the circumstances that communications are made via wired networks except communications between base stations and mobile terminals, and further that the distances between the base stations are set longer than the distance which radio waves are available.
However, in multi-hop wireless networks to which the present invention is applicable, direct wireless communications are required between neighboring base stations. Therefore, the distances between the base stations should be set shorter than the distance radio waves are available.
In such a case, since the distances between the base stations are short, the wireless signal emitted by the mobile station can reach to plural base stations. Therefore, even though a mobile station stays nearest to a current base station, the mobile station can communicate with other neighboring base stations.
Under these circumstances, the conventional handover control methods are used with the result that unnecessary handover occurs so often that problems arise such as increase in power consumption and wireless control signal. Moreover, wireless stations to which the invention is applicable are smaller in range where wireless signals are available than cellular phone systems, so that handovers tend to occur more frequently.
Besides, in case of cellular phone systems, since signals are transmitted almost all the time in a call, the level of received signals is always measured. By contrast, in case of mobile sensor networks, since the communication is not always made, handover control using reception signal levels requires base stations to frequently transmit reference signals for controlling the handover.